How is the thrust of the props converted to move the boat?

[Pinnacle]

Apologies, I don't think the title properly describes the question I am about to ask, but I couldnt think of a better way of putting it.......

So, on a twin screw mobo, the engines turn the shafts which in turn rotate the props. The props push water away from the boat ( in forward gear ) and the resistance to this is transmitted to the shafts.

So, my question is, what is it that transfers the forward thrust of the shafts into forward movement of the boat? Is it as simple as the engine mounts? For wxample on March II the forces must be massive to move c60 tons of GRP and metel at >20kts.

 

[MapisM]

Is it as simple as the engine mounts?

Traditionally, yes.
But nowadays, there are shafts whose coupling with the hull is made in such way that the thrust is taken directly by the hull.
Allowing in turn softer engine mounts to be used, for better vibrations absorption.
Not sure about M2 setup, though.

 

[Pinnacle]

Traditionally, yes.
But nowadays, there are shafts whose coupling with the hull is made in such way that the thrust is taken directly by the hull.
Allowing in turn softer engine mounts to be used, for better vibrations absorption.
Not sure about M2 setup, though.

So how does that work? I have a vision of some bearings with one part fitted to the shaft and ther other fixed to the shaft tube......

 

[[27631]]

on most modern fast boats...the engine mounts do take the total thrust from the prop shaft....scary really.!

 

[sarabande]

the prop shaft's connected to the thrust bearing,
the bearing's connected to the engine bearers,
the bearers are connected to the hull stringers,
the stringers are connected to the rest of of the boat,
Oh hear the word of the Lord !

 

[Pinnacle]

OK. But that must mean the thrust bearings ( and their mountings ) are mahoosive!

 

[MapisM]

Well, all the components must be adequate, of course.
But the thrust bearings aren't actually much bigger than a traditional stuffing box.
And the GRP (if that's what the boat is built of) structure is of course stronger in that area, but you would hardly notice that.
The diagram in this webpage gives a good general idea of how these shafts work.
http://www.aquadrive.com/system.html
There are other brands, with some differences, but the principle still stands.

As an aside, may I ask why you're interested?
Are big power engines tempting you, P?

 

[MapisM]

on most modern fast boats...the engine mounts do take the total thrust from the prop shaft....scary really.!

Define fast.
There's no such thing as a REALLY fast boat on traditional shafts.
Serious speed requires either surface drives, sterndrives, outboards or jets.
And with none of those the engine mounts take the thrust.

 

[Vitesse]

Does make me think about the transom on my Whaler with 200 odd kilos of outboard bouncing about...

 

[jfm]

Traditionally, yes.
But nowadays, there are shafts whose coupling with the hull is made in such way that the thrust is taken directly by the hull.
Allowing in turn softer engine mounts to be used, for better vibrations absorption.
Not sure about M2 setup, though.

+1. The thrust is taken (a) thru the engine mounts or (b) thru a thrust bearing near the top of the shaft that transmits the thrust to the hull at that point ie NOT via the engine

(b) allows softer engine mounts but requires a very strong mount for the thrust bearing, and the design of this increases stresses on the hull becuase of course such a mount cannot be as big as the 4 huge stringers that the engines sit on. Match 2 has (a). Let's calculate the forces on Match 2, very crudely:

1. Let's say the 1600hp engines make 1000hp each at 20kts cruise. So 1500kw (total for both), which is 1,500,000 joules per second of work (=energy) done per second
2. 20kts is 10.3 metres per second.
3. Force = work done/distance
4. Hence Force = 1500,000/10.3 = a force of 145,000 Newtons
5. Now with apologies to engineers and physicists, colloquially that that is 14.5 tonnes of force
6. The props are not perfect. Let's say they create 12 tonnes of forward force
7. Sanity check: do those numbers feel right? Yes, imho

So: we have 6 tonnes on each thrust bearing in the gearboxes. That's nowt. A lorry wheel bearing (for example) does more than 6 tonnes for thousands of hours. And we have 1.5 tonnes per engine mount. That too is nowt, I mean it's properly didly squat for a big engine mount and stringer. So in conclusion there are no difficult engineering challenges with taking the thrust through the engine and engine mounts

 

[jfm]

Does make me think about the transom on my Whaler with 200 odd kilos of outboard bouncing about...

The thrust force at sea is nowt compared with the acceleration forces induced when you drive over a pothole on the trailer

 

[Vitesse]

The thrust force at sea is nowt compared with the acceleration forces induced when you drive over a pothole on the trailer

I believe you. BTW, really enjoying your build thread. You have great enthusiasm and it's fascinating to watch the progress. Some of it I even understand.

 

[jfm]

Define fast.
There's no such thing as a REALLY fast boat on traditional shafts.
Serious speed requires either surface drives, sterndrives, outboards or jets.
And with none of those the engine mounts take the thrust.

Fast is the wrong measure - the relationship is if anything inverse. Take two boats with 3000hp. One is Match, doing 30kts. The other is some Miami Vice Fountain that does 100mph poker runs. The slower boat, Match, has MUCH higher thrust force

Remember from school that Power = force x speed? Hence, Force = power/speed. So AOTBE in terms of power, a faster boat has less forward thrust force than a slower boat.

 

[rafiki_]

I think the peak loads come from shock, for instance a rope/net snagging a prop and causing it to stop in a fraction of a second.

 

[jfm]

I think the peak loads come from shock, for instance a rope/net snagging a prop and causing it to stop in a fraction of a second.

No way. Those would be torsional loads only, not thrust which Pinnacle is asking about, and none of that torsional load is passed through the GRP hull or engine mounts. Those forces are all within the engine and drivetrain (and P bracket, depending on the details). Furthermore, rope/net is stretchy. Clouting a rock would cause a shock load, but not a net

 

[MapisM]

Fast is the wrong measure - the relationship is if anything inverse.

Agreed J, but aside from the fact that I was just replying to what sundancer said of "modern fast boats", what you're saying is 100% correct only in static conditions.
Actually, I'm not sure if "static" is the right word, but what I mean is, constant speed with constant drag - i.e. in waters as flat as a flat thing.
Now, if you just introduce the acceleration 100mph boats are capable of, things change radically.
Put even just some small waves into the equation, and things get even worse.
But it's when you throw in some jumps, with 20 meters long (or more) flights and the following re-entry, that the hell breaks loose with regard to the forces that the faster boat must withstand.
And not only forward force, but also sideways, upwards and even backwards!
That's what poker run boats (and even more so racing boats, of course) must withstand.
In fact, sometimes they don't. Returning home on just one outdrive, with the other one tore apart by a bad jump re-entry, is not that unusual.

Incidentally, that applies also to your reply to Vitesse.
He surely can't use his whaler for wavejumping at 100mph, but if he would, THAT would be the mother of all tests for his transom - much worse than trailering.
Though I agree that potholes can create much higher forces than any steady, "normal" cruising, of course.

 

[MapisM]

I think the peak loads come from shock, for instance a rope/net snagging a prop and causing it to stop in a fraction of a second.

Wrong example I'm afraid, 'cause as jfm pointed out that's torsional load.
But it's correct to say that also for thrust, peak loads come from shocks.
That's in a nutshell what I tried to explain in my previous post.

 

[Tranona]

No way. Those would be torsional loads only, not thrust which Pinnacle is asking about, and none of that torsional load is passed through the GRP hull or engine mounts. Those forces are all within the engine and drivetrain (and P bracket, depending on the details). Furthermore, rope/net is stretchy. Clouting a rock would cause a shock load, but not a net

But what ropes and nets can do is wind round the prop and shaft and pull the whole lot back, damaging the gearbox and even the mounts. Even more fun if the rope or net gets both props and pulls them together, pulling the P brackets out.

 

[jfm]

Agreed J, but aside from the fact that I was just replying to what sundancer said of "modern fast boats", what you're saying is 100% correct only in static conditions.
Actually, I'm not sure if "static" is the right word, but what I mean is, constant speed with constant drag - i.e. in waters as flat as a flat thing.
Now, if you just introduce the acceleration 100mph boats are capable of, things change radically.
Put even just some small waves into the equation, and things get even worse.
But it's when you throw in some jumps, with 20 meters long (or more) flights and the following re-entry, that the hell breaks loose with regard to the forces that the faster boat must withstand.
And not only forward force, but also sideways, upwards and even backwards!
That's what poker run boats (and even more so racing boats, of course) must withstand.
In fact, sometimes they don't. Returning home on just one outdrive, with the other one tore apart by a bad jump re-entry, is not that unusual.

Incidentally, that applies also to your reply to Vitesse.
He surely can't use his whaler for wavejumping at 100mph, but if he would, THAT would be the mother of all tests for his transom - much worse than trailering.
Though I agree that potholes can create much higher forces than any steady, "normal" cruising, of course.

Yup, all agreed MapisM. I wasn't meaning to argue with any earlier post you'd made btw, and sorry if it came across that way (forums eh?)
The forces when a jumping boat re-enters are high I agree, though I do not think there are any shock loads in the thrust direction that Pinnacle was asking about (ie axially along the drive shaft, if it were to be a shaft drive boat). Also I disagree that the thrust forces (or any forces I can readily think of) are higher during the (say) 10-20mph WOT acceleration phase than at steady state WOT 100mph. But once you introduce waves the forces get complex, and perhaps we dont want to go there in this thread?

 

[jfm]

But what ropes and nets can do is wind round the prop and shaft and pull the whole lot back, damaging the gearbox and even the mounts. Even more fun if the rope or net gets both props and pulls them together, pulling the P brackets out.

Sure, but that isn't what this thread is about afaik